PROJECT SUMMARY/ABSTRACT Numerous cerebellar malformations have been described in humans. Most cause cognitive, in addition to motor and sensory integration deficits. Surprisingly little is understood regarding the developmental basis of these malformations, particularly since little human specific data is available for normal or abnormal fetal cerebellar development. This proposal seeks to advance knowledge of human cerebellar development and malformations using human fetal samples and mouse models. The human-specific data will directly test the validity of our working mouse-derived hypotheses regarding the causes these disorders and strengthen the foundation of normal developmental data which will inform our ongoing genetic analyses of human cerebellar malformations. We will conduct the first in-depth analysis of normal human fetal cerebellar development from 4-23 Gestational Weeks, covering major developmental events. We will then examine the pathology of human fetal Dandy-Walker malformation the most common human cerebellar malformation, affecting ~1/3000 live births. Mouse models will be generated in conjunction with these experiments to assess the mechanisms of the developmental pathology. Finally, we will generate the first transcriptome data for normal human fetal cerebellar neurons. These cell-type specific data are critically missing from current publicly available brain resources. Our human fetal cerebellar neuron data will be compared to transcriptome data from existing datasets of endogenous mouse developing cerebellar neurons as well as mES and hPSC-derived cerebellar neurons to development to assess their validity as model systems. Further, the data will also be integrated with exome data from human cerebellar malformation patients to facilitate gene discovery for these important and understudied birth defects.